There is considerable interest in the using or recycling of ambient energy, which would normally be unused or lost, for example as waste heat, motion or sound. One field of interest relates to large-power generation, for example using wind or solar power. Another field of interest relates to small-scale power generation, for example for powering sensors.
In small-scale industrial or scientific applications, there is a desire to have self-powered sensors which store data and communicate it to a central monitoring station, for example wirelessly or by manual collection. Examples of uses of such sensors include body implants, climate or seismic activity monitoring, animal tracking, process control monitoring, engine management and remote displays. Such sensors are provided at least partly with power by scavenging from their environment, for example using waste energy in the form of heat transfer, solar radiation, material deformation and bio-fuel cells.
In body implants for medical uses such as pacemakers, the physically largest component is generally a battery, which may occupy as much as 50% of the internal volume of such an implant and which thus restricts the space available for electronics. It is desirable to reduce the battery size and/or increase the battery life. Known techniques for achieving this have relied on augmenting the power supply by means of energy derived from mechanical movement, cardiac activity and bio-fuel cells. In safety-critical applications where power must be available continuously, the risk associated with completely removing a battery is unacceptably high. For such applications, self-powered implants generally have to be capable of returning to battery power in the event of failure of a power generator or of insufficient ambient energy to supply the requirements.
For many types of sources of ambient energy, the scavenged energy is generally available in bursts, for example of pulses or alternating current cycles. Such bursts tend to have low repetition rates. For example, in the case of bio-medical scavenging, the repetition rate may typically be between 1 and 10 Hz. When several power generators are used, there is generally no fixed phase relationship between the supply of energy from the individual generators.
FIG. 1 of the accompanying drawings illustrates a typical power supply of known type for providing storage of scavenged energy. A plurality of scavenged energy power generators is connected to inputs 11-1N of the power supply. In the example shown in FIG. 1, the generators are assumed to be of the alternating current type and are connected via the inputs to corresponding full wave bridge rectifiers 21-2N. The outputs of the bridge rectifiers are connected in parallel across a storage capacitor 3 of sufficiently large capacitance to store sufficient charge for the intended application. One terminal of the capacitor 3 is connected via a diode 4 to a first terminal of a voltage—limiting diode 5, such as a Zener diode, and to the input of a voltage regulator 6, whose output forms the output 7 of the power supply. A back-up battery 8 is provided and has a first terminal connected via another diode 9 to the first terminal of the diode 5 and the input of the regulator 6. Second terminals of the capacitor 3, the diode 5 and the battery 8 are connected to a common line and to the common terminal of the regulator 6.
When the capacitor 3 is charged to a voltage greater than that of the battery 8, the diode 4 is forward-biased and conducts, whereas the diode 9 is reverse-biased and isolates the battery 8. The regulator 6 thus draws power from the charge stored in the storage capacitor 3 (and from any one generator providing sufficient voltage for its bridge rectifier to be conducting). Conversely, when the voltage across the capacitor 3 is below that of the battery 8, the diode 4 isolates the capacitor from the regulator input, which receives power from the battery 8.
The bridge rectifiers 21-2N are also such that the generator supplying the highest voltage at any time is connected to and charges up the capacitor 3 whereas the bridge rectifiers connected to the other generators are effectively reverse-biased and are unable to conduct. Thus, power generated by the other generators cannot be utilised for charging the capacitor 3 and hence is lost.
FIG. 2 of the accompanying drawings illustrates another known type of power supply comprising an input 1 for connection to a power generator, a full wave bridge rectifier 2, a storage capacitor 3 and a regulator 6 connected to the power supply output 7. The power supply further comprises a switch 10, for example in the form of a metal oxide silicon field effect transistor (MOSFET), for connecting or disconnecting the regulator to or from the capacitor 3 so as to switch the power supply on or off.
In use, the voltage across the capacitor 3 is monitored by means which are not shown in FIG. 2. When the voltage across the capacitor 3 rises to a sufficient value, the switch 10 is closed. Where the power supply forms part of a self-powered wireless sensor, for example, the sensor starts to operate and to transmit data. When the voltage across the capacitor 3 falls to an insufficient value, the switch 10 opens to allow the capacitor to be recharged by the power generator via the bridge rectifier 2. The measuring and transmission of data is therefore unpredictable and generally irregular, which may make such an arrangement unsuitable for many applications.
The power supplies shown in FIGS. 1 and 2 require a relatively large value of storage capacitance, in the form of a single capacitor or a plurality of parallel-connected capacitors, in order to store sufficient charge to supply the power for any significant or useful amount of time. However, this places a substantial stress on the generators, for example, when the capacitor 3 is fully discharged. In this state, the capacitor 3 substantially presents a short-circuit to the generators, which must therefore be capable of working into such a low load. This may also result in a relatively long charge-time. Also, each time the switch 10 of FIG. 10 closes, a high peak current demand may be made and this may apply a relatively large stress to the switch 10, which may raise an issue of reliability.